1. Field of the Invention
The present inventions pertain to semiconductor fabrication processing. More particularly, the present inventions relate to a system for reducing defectivity during semiconductor processing by maintaining a constant pH during the planarization process.
2. Description of the Prior Art
Referring now to FIGS. 1 and 2, there is shown a block diagram of a prior art CMP machine 100 and a side partial perspective view of a wafer 105 (FIG.2). The CMP machine 100 is fed wafers to be polished by an arm 101 and places them onto a rotating polishing pad 102. The polishing pad 102 is made of a resilient material and is textured, often with a plurality of predetermined grooves, to aid the polishing process. A conditioning arm 103 conditions the polishing pad.
The polishing pad 102 rotates on a platen 104, a turntable located beneath the polishing pad 102, at a predetermined speed. A wafer is held in place on the polishing pad 102 and the arm 101 by a carrier ring and a carrier film not shown. The lower surface of the wafer 105 rests against the polishing pad 102. The upper surface of the wafer 105 is against the lower surface of the carrier film of the arm 101. As the polishing pad 102 rotates, the arm 101 rotates the wafer 105 at a predetermined rate. The arm 101 forces the wafer 105 into the polishing pad 102 with a predetermined amount of down force. The CMP machine 100 also includes a slurry dispense tube 107, extending across the radius of the polishing pad 102. The slurry dispense tube 107 dispenses a flow of slurry 106 onto the polishing pad 102 from the slurry source 112. The slurry 106 is a mixture of deionized water and polishing agents designed to aid chemically the smooth and predictable planarization of the wafer. In a system using silica slurry the pH of the slurry is very high, typically having a pH of around 10 or 11.
The rotating action of both the polishing pad 102 and the wafer 105, in conjunction with the polishing action of the slurry, combine to planarize, or polish, the wafer 105 at some nominal rate. The polishing action of the slurry is comprised of an abrasive frictional component and a chemical component. The abrasive frictional component is due to the friction between the surface of the polishing pad, the surface of the wafer, and abrasive particles suspended in the slurry. The chemical component is due to the presence in the slurry of polishing agents which chemically interact with the material of the dielectric or metal layer of the wafer. The chemical component of the slurry is used to soften the surface of the dielectric layer to be polished, while the frictional component removes material from the surface of the wafer.
Slurry dispense termination is accomplished by turning off a pump, which will stop the flow of slurry onto the pad. After the slurry dispense process is terminated, deionized water having a pH of about 6 is dispensed from the deionized water source 110 via the water dispense tube 108 onto the pad. The wafer substrate is then rid of the slurry, and the pH at the surface of the wafer is lowered to a pH of about 6.
A typical chemical mechanical polish cycle includes two or more linear or rotary tables that do the planarizing and then a third table that does a water buff on a softer pad. During the chemical mechanical planarization (CMP) process, a wafer undergoes many pH changes. Slurry is used to first polish the wafer and then dionized water is used to clean the wafer afterwards. The pH is raised during the polish steps, then lowered by deionized water whenever the wafer is to be kept wet. At post-CMP cleans, the pH is also raised for cleaning then lowered as the wafer is rinsed and dried. This amounts to many fluctuations between a pH of approximately 11 and a pH of approximately 6.
Referring now to FIG. 3, there is shown a block diagram of one example of a prior art typical CMP process 200. To prevent slurry from drying on the wafer, the wafer surface is kept wet through-out the clean-up process. A wet transfer input station mechanism 210 is used to load and unload the wafers while keeping the surface of the wafer wet with deionized water. The wafer 105 is then transferred to polishing platen 220, where a high pH slurry polish is followed by an automatic rinse of deionized water, once the polish is complete. The wafer is then transferred to secondary polishing platen 230, where a second high pH slurry polish is again followed by a deionized water rinse, when the secondary polish is complete. Typically, the wafer is transferred to a third, softer platen 240, where the wafer is buffed on the pad while deionized water is sprayed on the pad. Typically, the above three platens, 220, 230 and 240 are included on the same multiple platen CMP machine 205.
The wafer 105 is then unloaded and kept wet using the input station mechanism 210. Then, optionally, the wafer 105 may be transferred to brush stations 250 and 255, where the wafer is brushed with a scrub solution spray having a pH similar to that of the slurry, such as dilute ammonia, or NH4OH. Finally, the wafer 105 is transferred to the drying station 260, where it is rinsed with deionized water and dryed.
FIG. 4 is a table showing the pH level of the wafer surface at the end of each polishing step of the system 200 of FIG. 3. Note that the pH at the surface of the wafer fluctuates numerous times between a pH of approximately 11 and a pH of approximately 6, during the CMP process.
All particulate matter develops an electrically charged thin layer when suspended in a liquid solution. This charge is known as the zeta potential and can be either negative or positive. The zeta potential appears at the outer surface of the particle such that the particle is surrounded by a small charge field. Silica particles in a basic aqueous solution having a pH of about 10 or more results in a negative zeta potential on the silica particles. In addition, the zeta potential of any other particles present, as well as that of the surfaces contacted by the solution, is negative at such a high pH. The silica particles are thus electrostatically repelled from the semiconductor wafer facilitating the removal of the slurry residue from the wafer surface. When the pH at the surface of the wafer is lowered in the presence of silica particles, colloids form and silica agglomeration occurs on the surface of the wafer. As such, any time the pH of the wafer surface is lowered, a higher defectivity environment exists in the presence of microscopic particles. Defects generated include scratches on the wafer by slurry abrasive agglomerates and slurry abrasive (or any other particle) attaching to the wafer surface.
What is needed is a method for providing a constant, high pH through the entire CMP polish and clean process to minimize slurry abrasive agglomeration and fluctuations of pH.
This object, and others, is satisfied by Applicant's present inventions disclosed herebelow.